As a plated steel sheet with a good corrosion resistance, there is a galvannealed steel sheet. This galvannealed steel sheet usually is produced by degreasing the steel sheet, then preheating it in a non-oxidizing furnace, reduction annealing it in a reducing furnace to clean the surface and secure the quality, dipping it in a hot-dip galvanizing bath, controlling the amount of deposition, then alloying. Due to its characteristics of superior corrosion resistance, plating adhesion, etc., it is widely used for automobile, building material, and other applications.
In particular, in recent years, in the automobile sector, galvanized steel sheets have to be made higher in strength to achieve both the function of protecting passengers at the time of collision and reducing weight for the purpose of improving fuel efficiency.
Further, recently, to make the reaction at the surface of the steel sheet at the time of annealing more uniform and improve the plating appearance, production systems for galvanized steel sheet using all radiant tube type annealing furnaces have spread in use.
To make the steel sheet higher in strength without reducing the workability, addition of elements like Si, Mn, and P is effective. These elements are selectively oxidized in the reduction annealing step and become concentrated at the surface of the steel sheet. In particular, oxides of Si concentrated at the surface of the steel sheet cause the wettability of the steel sheet and molten zinc to drop. In extreme cases, the molten zinc will not adhere to the steel sheet.
Therefore, to plate with molten zinc a steel sheet to which an element like P has been added, use has been made of the method of making the thickness of the oxide film of the iron a suitable range to keep down the production of oxide layers of elements such as Si, Mn, and P and improve the wettability (for example, see Japanese Patent No. 2513532) or the method of pre-plating to improve the plating wettability (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2-38549).
Further, the inventors proposed the method of production comprising suitably controlling the reducing atmosphere to cause internal oxidation of SiO so as to improve the plating wettability (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2001-323355).
However, the technology disclosed in Japanese Patent No. 2513532 and Japanese Unexamined Patent Publication (Kokai) No. 2001-323355 is technology using a Sendzimir type hot-dip galvanizing steel sheet production system for heating in a non-oxidizing atmosphere and annealing in a reducing atmosphere and cannot be used in a manufacturing equipment of hot-dip galvanized steel sheet using an all radiant tube type annealing furnace. Further, in the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-38549, a pre-plating system is necessary. When there is no installation space, it cannot be used. Further, a rise in cost due to the installation of the pre-plating system is unavoidable.